This application claims the benefit of Korean Patent Application No. 1999-11108, filed on Mar. 31, 1999, and the benefit of Korean Patent Application No. 1999-48411, filed on Nov. 3, 1999, each of which are hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a reflective LCD device including a cholesteric liquid crystal (CLC) color filter.
2. Description of Related Art
In general, LCD devices are divided into reflective LCD devices and transmissive LCD devices. The transmissive LCD device uses an internal light source such as a back light, while the reflective LCD device uses ambient light.
Particularly, since the reflective LCD device uses ambient light, the brightness of the display depends on circumstances. In an office, the reflective LCD device is lower in brightness than the transmissive LCD device and, accordingly the color purity of an absorption-type color filter used in the LCD should be sacrificed to increase the brightness.
FIG. 1 is a cross-sectional view of a conventional reflective liquid crystal display.
As shown in FIG. 1, the liquid crystal panel includes a linear polarizer 26, a retardation film 24, a diffuser film 22, a first substrate 10, a color filter 20, a common electrode 18, a liquid crystal layer 16, a reflective electrode 14 and a second substrate, each are stacked in the above-described order.
The reflective electrode 14 reflects light transmitted from outside the display and also functions as a pixel electrode. The reflective electrode 14 and the common electrode 18 apply a voltage to the liquid crystal layer 16 and change the orientation of liquid crystal molecules. The diffuser film 22 reduces a surface reflection of light and increases a viewing angle. The retardation film 24 such as a xcex4 plate converts linearly polarized light into circularly polarized light. Further, the linear polarizer changes the natural light into linearly polarized light.
The reflective LCD device described above functions and acts as follows.
When natural light is incident into the LCD device, the natural light is converted into linearly polarized light by the linear polarizer 26, then converted into circularly polarized light by the retardation film 24. The circularly polarized light is converted into linearly polarized light while passing through the liquid crystal layer 16 and is reflected on the reflective electrode 14. The reflected polarized light is converted into circularly polarized light while passing through the liquid crystal layer again, then passes through the color filter to produce colored light.
The circularly polarized light is diffused to increase the viewing angle while passing through the diffuser film 22, then is converted again into linearly polarized light while passing through the retardation film 24. The linearly polarized light is displayed to the user after passing through the linear polarizer 26 in the form of images.
FIG. 2 shows the state of light while it passes through each of the components described above when an electric field is not applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer 26. The linearly polarized light is changed into circularly polarized light through the retardation film 24. The circularly polarized light is converted again into linearly polarized light through the liquid crystal 16, then reflected by the reflective electrode 14. The reflected linearly polarized light is changed into circularly polarized light through the liquid crystal layer 16. The circularly polarized light is finally converted into linearly polarized light through the retardation film 24.
FIG. 3 shows the state of light while it passes through each of the components described above when an electric field is applied to the liquid crystal layer.
The natural light is first converted into linearly polarized light through the linear polarizer 26. The linearly polarized light is changed into circularly polarized light through the retardation film 24. The circularly polarized light is not changed when passing through the liquid crystal 16 as an electric field is applied to the liquid crystal 16, then reflected by the reflective electrode 14. The reflected circularly polarized light is not varied even when passing through the liquid crystal 16. The circularly polarized light is finally converted into linearly polarized light through the retardation film 24, then absorbed by the linear polarizer 26.
FIG. 4 is a graph illustrating the reflectivity of light with respect to the incident light of the LCD device described above. In FIG. 4, the X-axis indicates a wavelength xcex, and the Y-axis indicates a reflectivity. Note that a dominant wavelength region is referred to as region A and other wavelengths are referred to as region B. As shown in the graph, though light""s reflective index is relatively high in the region A, because light reflection is also carried out in the region B, the color purity of the LCD is reduced. It is required that the color purity is reduced in order to increase the transmissivity of the color filter, but just lowering the color purity to increase the brightness has a limitation.
Further, since the LCD having the configuration described above has a multi-layered structure in which each layer, i.e., each component differs from one another in reflective index, the intensity of the light is reduced while the light passes through each component. For example, the intensity of the light first is reduced while passing through the linear polarizer 26, and then also prominently is reduced after passing through the color filter 20, because part of the light is absorbed or reflected while passing through the color filter 20.
Further, though the observer can clearly see the image displayed due to a good contrast ratio in the center of the screen, the contrast ratio becomes lower as it gets far from the center of the screen, thereby deteriorating the display characteristic.
An object of the present invention is to provide a reflective liquid crystal display device having increased brightness without sacrificing color purity.
Another object of the present invention is to provide a reflective liquid crystal display device having high color purity and an improved contrast ratio.
In order to achieve the objects, in a first embodiment, a reflective liquid crystal display including a linear polarizer for converting natural light into linearly polarized light; a retardation film for converting the linearly polarized light into a circularly polarized light; a liquid crystal layer for varying the phase of the light differently depending on the presence of an electric field; a cholesteric liquid crystal color filter for selectively reflecting the light from the liquid crystal layer; and a black background for absorbing the light passing through the color filter.
The present invention also provides, in the first embodiment, a reflective liquid crystal display including first and second substrates opposite to and spaced apart from each other; a liquid crystal layer interposed between the first and the second substrates, the liquid crystal layer having a first switching mode in which the phase of light is changed while passing through it and a second switching mode in which the phase of light is not changed while passing through it; first and second electrodes for applying an electric field to the liquid crystal layer; a semiconductor element located on the second substrate, for switching an electric signal applied to the liquid crystal layer; a retardation film located on the first substrate, for converting a linearly polarized light a circularly polarized light; a linear polarizer located on the retardation film, for converting natural light into the linearly polarized light; a cholesteric liquid crystal color filter located on the second substrate, for selectively reflecting the light from the liquid crystal layer as a light of at least one color; and a black background located beneath the second substrate, for absorbing the light passing through the color filter.
The retardation film is a xcex/4 plate. The black background is located beneath the color filter. The retardation film is located between the linear polarizer and the color filter. The black background is made of a polymeric material. The color filter is designed so that a wave bandwidth of the color filter can be controlled by adjusting the pitch of the cholesteric liquid crystal.
The present invention also provides, in a second embodiment, a reflective color liquid crystal display device, including first and second substrate, spaced apart from and opposite to each other; a liquid crystal layer interposed between the first and second substrates and having liquid crystal molecules and a xcex/4; phase difference a linear polarizer arranged over the first substrate, the polarizer converting natural light into linearly polarized light; a retardation film arranged under the linear polarizer, the retardation film converting the linearly polarized light into circularly polarized light; a negative uniaxial film arranged between the retardation film and the liquid crystal layer, the uniaxial film compensating a phase difference between a direction of an incident light entering the liquid crystal layer and a direction of the liquid crystal molecules adjacent to the first and second substrates of the liquid crystal layer; a cholesteric liquid crystal color filter arranged under the liquid crystal layer, the color filter selectively reflecting the light from the liquid crystal layer; and a black background arranged under the color filter, the black background absorbing the light passing through the color filter.
The liquid crystal molecules have a homeotropic orientation when an electrical field is not applied.
The present invention also provides, in a third embodiment, a reflective color liquid crystal display device, including a first substrate; a second substrate spaced apart from and opposite to the first substrate, the second substrate including a plurality of pixel electrodes, and a common electrode being spaced apart from each other and being arranged thereon; a linear polarizer changing natural light into linearly polarized light and being positioned at an outer surface of the first substrate; a liquid crystal layer interposed between the first and second substrates and having liquid crystal molecules being oriented by a parallel electric field between the pixel electrode and the common electrode; a cholesteric liquid crystal color filter disposed between the liquid crystal layer and the second substrate, selectively reflecting the light from the liquid crystal layer; and a black background absorbing the light passing through the color filter.
The reflective color liquid crystal display device further includes a negative uniaxial film arranged on the liquid crystal layer.